Multiple beam ellipsometer

ABSTRACT

An ellipsometric apparatus provides two impinging focused probe beams directed to reflect off the sample along two mutually distinct and preferably substantially perpendicular directions. A rotating stage rotates sections of the wafer into the travel area defined by two linear axes of two perpendicularly oriented linear stages. As a result, an entire wafer is accessed for measurement with the linear stages having a travel range of only half the wafer diameter. The reduced linear travel results in a small travel envelope occupied by the wafer and, consequently, a small footprint of the apparatus. The use of two perpendicularly directed probe beams permits measurement of periodic structures along a preferred direction while permitting the use of a reduced motion stage.

CLAIM OF PRIORITY

The present application is a continuation of U.S. patent applicationSer. No. 10/893,449, filed Jul. 16, 2004, now U.S. Pat. No. 6,985,228which is a continuation of U.S. patent application Ser. No. 10/042,592,filed Jan. 9, 2002, now U.S. Pat. No. 6,798,512 which claims priority toU.S. Provisional Patent Applications Ser. No. 60/311,035, filed Aug. 9,2001, and Ser. No. 60/336,437, filed, Nov. 1, 2001, each of which ishereby incorporated herein by reference.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to ellipsometric systems for measuringfeature configurations on wafers. Particularly, the present inventionrelates to a compact ellipsometer having multiple measurement beams. Themeasurement beams operate alternately and in conjunction with a rotatingstage such that linear stages for positioning the wafer may be operatedwithin reduced travel ranges.

BACKGROUND

Semiconductors are typically fabricated by depositing and etching anumber of layers that are shaped and configured on the upper or topsurface of a wafer. Controlling those fabrication steps and testing thewafer early during production helps to keep production costs low. Anincreasingly important technique for a non-destructive measurement ofsemiconductors is ellipsometry. In ellipsometry, a specificallyconfigured probe light beam is directed to reflect off the wafer. Thechange in polarization state of the beam induced by the interaction withthe wafer is monitored to provide information about the wafer.

Ellipsometers have been used extensively to monitor thin film parameterssuch as thickness, index of refraction and extinction coefficient. Morerecently, ellipsometers have been used to monitor the properties(critical dimensions) of small, repeating, periodic structures onwafers. These periodic structures are similar to a grating and themeasured data can be subjected to a scatterometry analysis to deriveinformation about the structure. Information of interest includes, butis not limited to, line width and spacing as well as sidewall profile.

Such periodic structures have distinct orientations. It has been foundthat the most useful information about such structures can be obtainedif the probe beam of the ellipsometer is directed substantiallyperpendicular to the line structure.

As seen in FIG. 1, a typical wafer W will have multiple such periodicstructures PS formed thereon. In some cases, all of the periodicstructures will be oriented in the same direction (i.e. all linesparallel). In other cases, some of the structures will have linesrunning perpendicular to other structures.

A conventional, ellipsometer is typically provided with a stage formoving the wafer through full linear motions FX, FY as well as rotationabout the central axis so that the probe beam PB can be directed to eachof the periodic structures PS in the appropriate direction (usuallyperpendicular to the line structure). The linear motions FX, FY areabout equal to the wafer diameter WD. The wafer W moved during themeasurement consequently occupies a travel envelope LE that extends inthe directions of each of the linear axes about twice the waferdiameter. The travel envelope LE determines the minimal footprint of anellipsometer apparatus.

Recently, there has been a push to substantially reduce the size ofellipsometer apparatus. This effort is particularly directed to allowingan ellipsometer to be incorporated directly into a semiconductorprocessing tool. To achieve the desired miniaturization, stage systemhave been developed which reduce the total range of motion of the wafer,thereby reducing the travel envelope and consequently the footprint ofthe system (e.g. stages that implement a cylindrical coordinate systemconsisting of a linear and a rotational stage). The use of these stageshas not significantly impeded the measurement of thin film parameterssince such measurements are not effected by the direction in which theprobe beam strikes the sample. However, such reduced motion stagesystems have caused a problem with measuring periodic structures wherethe impinging direction of the probe beam PB has to correspond to ameasurement relevant orientation of the periodic structure PS.

As shown in FIGS. 2A and 2B the wafer can be manipulated with the X- andY-stages such that only one of the four quadrants of the sample islocated at the intersection between the sample and the probing beam. Toperform measurements within the other three quadrants of the sample, therotating stage needs to move by multiples of 90°. To achieveperpendicular orientation of the periodic sample structures in thesecases, a second probing beam perpendicular to the first one isnecessary.

This difficulty can best be seen in FIGS. 2A and 2B. In FIG. 2A, theprobe beam PB is shown striking periodic structure PSI perpendicular tothe line structure. When the operator wishes to measure periodicstructure PS2, the rotating stage is used to bring the sector of thewafer where that structure is located within the region which can bereached by the probe beam. As noted above, the stage travel in the X andY directions is not sufficient to bring the structure PS2 under theprobe beam without a rotation. Unfortunately, and as seen in FIG. 2B,the result of this rotation is to orient the periodic structure PS2 sothat the probe beam impinges thereon in a direction parallel to thelines. As noted above, it has been found that most relevant informationcan be obtained when the beam strikes the structure perpendicular to theline structure.

Accordingly, it would be desirable to develop an ellipsometer system,which can utilize a reduced motion stage but also provides for optimalmeasurement of both thin film parameters and periodic structures.

BRIEF SUMMARY

In the present invention, a probe beam is selectively directed along twoor more beam paths to provide two or more focused beams impinging invarious impinging directions on a tested wafer. Two perpendicularlyoperating linear stages provide a travel area that is only a fraction ofthe wafer size. Combined with the linear stages is a rotating stagepositioned with its axis of rotation perpendicular to the movement planedefined by the linear stages. The movement plane is preferably parallelto the top of the fixed wafer. The rotating stage rotates the fixedwafer within a rotation range such that a number of sectors of the wafertop are brought within range of the respective focal spot duringconsecutive sector measurement steps. During a sector measurement step,only the linear stages are operated to move the wafer along the focalspot.

The focused beams are positioned in a number and in an angularorientation to each other that corresponds to the number of angularorientations of patterns within the measurement sectors. In thepreferred embodiment, where wafer patterns have one measurement relevantorientation, two focused beams are provided in perpendicular impingingdirections relative to each other such that the pattern can be measuredin all four quadrants while still maintaining a perpendicularorientation of the pattern relative to the plane of the probing beam.Since two focused beams are utilized, the rotating stage operates onlyto adjust the wafers global orientation prior to the sector measurementsteps and to rotate the predetermined sectors within the travel area sothat is accessible by the focal spots. In the preferred embodiment, foursectors are defined for measuring a wafer in four consecutive sectormeasurement steps. The linear ranges of the linear stages are about halfthe wafer diameter, which significantly reduces the travel envelope andconsequently the footprint of the apparatus.

The goal of the subject invention could be achieved using two completelyseparate ellipsometers mounted on the same support system. In otherwords, two light sources, two sets of focusing and collecting optics,two sets of polarizers and analyzers and two separate detectors could beused. However, in the preferred embodiment, only a single light sourceand a single detector are used. This approach not only conserves space,but also reduces complexity as only one light source and one detectorneeds to be adjusted and characterized for the measurement.

Preferably, a broad band light source is used to generate apolychromatic probe beam. At some point before striking the sample,optics are provided for either splitting the beam along two paths orselectively directing the beam along a first or a second beam path.After reflection off the sample, optics are provided before the detectorto either recombine the previously split beam portions or selectivelycombine the two beam paths into a single path. In the preferredembodiment, movable mirrors are used to create two beam paths ratherthan splitting the beam to maximize the light energy being used for themeasurement.

The subject invention is not limited to any particular ellipsometerconfiguration. Those skilled in the art will be aware of many variantssuch as rotating polarizer (analyzer) or rotating compensator systems.The elements necessary to create the polarization state of the incomingprobe beam and analyze the polarization state of the reflected beam canbe located in the common path regions or in the separate path regions.If located in the separate path regions, two sets of optical elementsare needed as shown in the preferred embodiment illustrated herein.

In the illustrated embodiment, a broadband rotating compensator(waveplate retarder) system is shown. Such a system is disclosed in U.S.Pat. No. 5,973,787. A suitable rotating analyzer system is shown in U.S.Pat. No. 5,608,526. See also, U.S. Pat. No. 6,278,519. All of the abovepatents are incorporated herein by reference.

In the illustrated embodiment, a pair of movable mirrors is provided forcontrolling the beam propagation. In a first position, the mirrors arelocated out of the beam path and allow the beam to travel along a firstbeam path. In a second position, both mirrors are located within thebeam path and cause the beam to travel along the second beam path. Themoveable mirrors are specifically configured to provide high positionaccuracy and repeatability for a large number of switching cycles.

In the preferred embodiment, stepper motors that have a hollow shaft areutilized to rotate and control the waveplates. The stepper motors areplaced such that the beam paths run through the hollow shaft of thestepper motors' axes of revolution. The hollow shaft assembly reducessignificantly the space otherwise occupied by the mechanism for drivingthe waveplates contributing to a reduced footprint of the opticalassembly. As a result, the optical assembly fits into the apparatusdespite the increased number of individual optical components.

The combination of single light source and single detector incombination with reduced travel range, multiple alternate focused beams,hollow shaft waveplate assembly and moving mirrors provides for anellipsometry apparatus that has a footprint smaller than the travelenvelope LE for a given wafer size.

As noted above, the subject invention allows periodic structures to bemeasured from a preferred direction while using a reduced motion stage.It should also be noted that this system allows any measurement area tobe measured from any direction. While it is generally true that maximuminformation may be obtained when measuring a periodic structure when thebeam is directed perpendicular to the line structure, additionalinformation may be obtained from measurements where the beam is alsodirected parallel to the line structure or even at a 45 degree anglewith respect thereto. In cases where the measured structure is rathersimple, this information alone might even be sufficient. The subjectinvention allows measurements from any desired direction. Suchmeasurements could be used individually or combined in a regressionanalysis to more fully characterize the structure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a simplified scheme of a prior art positioning system of anellipsometer system where two linear stages move a wafer in ranges aboutequal the wafer diameter.

FIGS. 2A and 2B show simplified schemes of a prior art positioningsystem of an ellipsometer system where two linear stages move a wafer inranges about half the wafer diameter. In such prior art positioningsystem the probe beam is limited to direction irrelevant reflectancemeasurements.

FIG. 3 shows a first perspective view of an exemplary apparatusaccording to the preferred embodiment of the present invention.

FIG. 4A shows a top view of the device of FIG. 3 as indicated in FIG. 3by the arrow 17. The device is shown with a light beam propagating alonga first beam path.

FIG. 4B shows schematic and simplified the main contents of FIG. 4A forthe purpose of general understanding.

FIG. 5A shows a top view of the device of FIG. 3 as indicated in FIG. 3by the arrow 17. The device is shown with the light beam propagatingalong a second beam path.

FIG. 5B shows schematic and simplified the main contents of FIG. 5A forthe purpose of general understanding.

FIG. 6 shows a second perspective view of the device of FIG. 3 with thelight beam propagating along the first beam path.

FIG. 7 shows the second perspective view of the device of FIG. 3 withthe light beam propagating along the second beam path.

FIG. 8 shows the second perspective view with the device of FIG. 3having a top portion removed. A travel envelope occupied by a work pieceduring the operational use of the device is illustrated. Also shown is atravel area provided by the travel ranges of the linear stages.

FIG. 9 shows the second perspective view with the device of FIG. 3having the top portion removed. A first focused beam is illustratedimpinging a first sensitive measurement area in a first impingingdirection.

FIG. 10 shows the second perspective view with the device of FIG. 3having the top portion removed. A second focused beam is illustratedimpinging a second sensitive measurement area in a second impingingdirection.

FIG. 11 is a perspective view of a hollow shaft stepper motor used tomount a rotating waveplate.

DETAILED DESCRIPTION

According to FIG. 3, an ellipsometric apparatus 1 of the preferredembodiment is configured in order to make directional reflectance (inthis embodiment, ellipsometric) measurements on the top 9 (see FIGS. 4A,5A, 6–10) of a wafer 2. The ellipsometric apparatus includes a base 5and a top portion 8, which carries an optical assembly 12. The opticalassembly 12 includes two rotating waveplate assemblies 10, 14.

Directional reflectance measurements are measurements for which acertain impinging direction of a focused beam relative to structure 6,11 (see FIGS. 4A–10) needs to be maintained. The structures 6, 11 may bepatterns as they are well known to those skilled in the art. Thestructures 6, 11 typically have a plurality of parallel lines in agrating-like configuration. The structures 6, 11 may also be layerconfigurations and other features on the wafer top 9 with propertiesmeasurable by the known techniques of ellipsometry. For more detailedinformation on related ellipsometry techniques, see the patents citedabove.

It is noted that for the purpose of general understanding, the elementsillustrated in the figures are schematically shown without any claim foraccuracy. Moreover, for the purpose of clarity, propagating beams areshown as solids.

The ellipsometric apparatus 1 has a base 5 on which a first linear stage4 is assembled. On top of the first stage 4 is mounted a second linearstage 3, which itself carries a rotating stage 7. The first stage 4 mayhave a first travel range TX (see FIG. 8) along a first linear axis Aland the second stage 3 may have a second travel range TY (see FIG. 8)along a second linear axis A2 perpendicular to axis A1. First stage 4and second stage 3 are positioned and computer controlled operated suchthat the rotating stage 7 may be moved within the apparatus as shown bya travel area TA (see FIGS. 8–10).

The travel area TA is defined by the ranges TX, TY. The travel area TAis the area accessibly by focal spots 41, 81 (see FIGS. 4A–7, 9, 10) byonly operating the linear stages 3, 4. The travel area TA is a fictiveand global entity introduced for the purpose general understanding. Itcan be imagined as being drawn by the focal spots 41, 81 on an fictiveelement directly attached to the linear stage 3 while the linear stages3, 4 move within their travel ranges TX, TY.

The rotating stage 7 has a rotating range preferably of 360 degreesaround an axis of revolution AR (see FIG. 8). The rotating stage 7 isfixed on the linear stage 3 such that any area of the concentricallysupported wafer 2 may be brought within the travel area TA by rotatingthe wafer 2 via the rotating stage 7.

In the preferred embodiment, the wafer 2 has a wafer diameter WD (seeFIG. 8) that is about twice the amount of the ranges TX, TY. The firstrange TX defines together with the diameter WD a first envelopeextension EX (see FIG. 8). The second range TY defines together with thediameter W) a second envelope extension EY (see FIG. 8). Extensions EX,EY define a travel envelope 90 (see FIG. 8).

Reflectance measurements are performed on essentially the whole wafertop 9 in a number of consecutive measurement steps where individualwafer sectors 91–94 (see FIGS. 9, 10) are brought within the travel areaTA. Following the step of positioning one of the sectors 91–94 withinthe travel area TA, the wafer 2 is linearly moved by the stages 3, 4 tobring predetermined test areas within the focal spots 41, 81. The focalspots are fixed and defined by the optical assembly 12. A predeterminedtest area may be part of the structures 6, 11, which have differentmeasurement relevant orientation on the wafer top 9. Structures 6, 11are solely shown for the purpose of general understanding to exemplarilyrepresent the dense arrayed patterns that are measured by the apparatus1. The structures 6, 11 are direction sensitive measurement areas, whichrequire a predetermined impinging direction in order to accomplish areflectance measurement in accordance with known techniques ofellipsometry. A multitude of such direction sensitive measurement areasmay be present on a wafer top 9 with varying angular fabricationorientation.

The apparatus top 8 is dimensioned in correspondence with the base 5 andmay extend beyond the footprint of the base 5 for the purpose ofproviding secured access to supply and communication cables (not shown)as is clear to one skilled in the art. In the preferred embodiment, acommon light source and a single final detector are utilized in orderfor the optical assembly 12 to fit within the apparatus top 8 togetherwith eventual other functional elements like, for example a well knownfocusing unit and/or calibration unit (not shown). The apparatus top 8is positioned in a gap height HG above the rotating stage 7 such thatthe rotating stage 7 is externally accessible for placing the wafer 2 onit.

In the preferred embodiment, the optical assembly 12 is configured toprovide a first focused beam 40 (see FIGS. 3, 4A, 4B, 6, 9) andalternately a second focused beam 80 (see FIGS. 5A, 5B, 7, 10) from aninitial light beam 30 (see FIGS. 3–7). The first focused beam 40provides a first focal spot 41 in a first impinging direction (see FIG.9). The second focused beam 80 provides a second focal spot 81 in asecond impinging direction (see FIG. 10). Impinging directions are thedirections of center axes of the focused beams 40, 80.

A first moveable mirror 28 (see FIGS. 4A–7) alternately switches theinitial light beam 30 between a first beam path and second beam path.According to FIG. 4A, 4B, the first beam path includes the initialsection 31 up to a first lens unit 37 from which the first focused beam40 propagates towards the first focal spot 41. The first beam pathfurther includes a first inclining path segment 46 from a parabolicmirror 45 to the mirror 33 and a path end segment 50 from the mirror 33up to a parabolic mirror 56.

According to FIGS. 5A and 5B, the second beam path includes the initialsection 71 from a first moveable mirror 28 being in an in-position suchthat it redirects the initial light beam 30 up to a second lens unit 77from which the second focused beam 80 propagates towards the secondfocal spot 81. The second beam path further includes a second incliningpath segment 86 from a parabolic mirror 85 to the mirror 73 and a pathend segment 70 from the mirror 73 up to a second moveable mirror 54. Thesecond moveable mirror 54 is also in an in-position where it redirectsthe incoming light beam towards the parabolic mirror 56. Light beamspropagating along first or second beam path are both directed towardsfinal optical elements, which prepare a terminating beam 60 to terminateon a single detector 61.

The final optical elements include the parabolic mirror 56 together withmirrors 57, a pin hole 58 and a holographic grating 59, which prepare ina well known fashion the terminating beam 60 for impinging andterminating in the final detector 61. In the preferred embodiment, theparabolic mirror 56 has a focus angle of 20° and a focal length of 100mm, the holographic grating 59 is from Jobin Yvon, part number543.02.190 and the final detector 61 is a CCD detector having 512pixels, each corresponding to a different narrow wavelength bandwidth.The pin hole 58 includes a selectable element for changing the size ofthe pin hole so that the measurement spot size can be varied.

The scope of the invention includes embodiments in which other opticalfeatures well known for alternately redirecting an incoming light beamare used instead of the moveable mirrors 28, 54. The scope of theinvention is also not limited by a specific mode by which the moveablemirrors 28, 54 alternately redirect the incoming beams. For example, aconfiguration may be selected in which one of the moveable mirrors 28,54 is in an in-position while the other one is in an out-position whereit does not interfere with a light beam. The scope of the invention isalso not limited by a particular shape of geometry of the beam paths orby any particular number and/or configuration of additional opticalelements like, for example, mirrors 34, 74.

According to FIG. 4A, a first structure 6 has a measurement relevantorientation on the wafer top 9 requiring a first impinging directionprovided by the first focused beam 40. In order to perform thereflectance measurement in accordance with the known techniques ofellipsometry, the first focused beam 40 impinges the structure 6 withinthe first focal spot 41 along the first impinging direction andinitiates a first reflected beam 42 whose polarization state has beenchanged away from the focal spot 41. The first focused beam 40 isprovided by a first lens unit 37 and a first polarizer 35, which focusand polarize the initial light beam 30 propagating along the first beampath. In the preferred embodiment, the first lens unit 37 is a tripletlens with f=69 mm and the polarizer 35 is a Rochon prism. A mirror 32 isspatially oriented to redirect the propagating beam from a horizontalbeam plane towards the angulated oriented and lower positioned polarizer35 and lens unit 37. The beam plane is sufficiently high above the top 8to give room for mechanical features used, for example, for positioningand fixating the optical elements on the top 8.

The initial light beam 30 is provided by a light source, which mayinclude but is not limited to a white light source 20, a lens 21, a UVlight source 22, an ellipsoid mirror 23, a source pinhole 24 and aparabolic mirror 25. In the preferred embodiment, the white light source20 is a tungsten light bulb, the UV light source 22 is a D2 UV-lamp, theellipsoid mirror 23 has f=80 mm, and the parabolic mirror 25 has focusangle of 20° and f=50 mm.

To obtain an ellipsometric measurement with high accuracy, polarizationdetection needs to be performed on the reflected beams 42, 82 (see alsoFIGS. 7, 10) with only a minimum number of additional reflectionsinduced on the reflected beams 42, 82. Since the reflected beams 42, 82propagate conically away from the focal spots 41, 81, at least oneoptical element is used to collimate the reflected beams 42, 82. Thecollimating optic can be a lens, or as shown in the illustratedembodiment, parabolic mirrors 45, 85, which have in the preferredembodiment has a focus angle of 45° and f=59.51 mm.

In the present invention, polarization detection is performed byrotating the waveplate (under computer control) around an axis ofrevolution, which is parallel to the propagation direction of the beampassing through the waveplate with the beam centered on the rotationsymmetry axis. In the present invention, the polarization detection isat a high level of accuracy by introducing two alternately operatingrotating waveplate assemblies 10, 14 such that each of the two reflectedbeams 42 is passed through one of the two rotating waveplates with onlya single prior reflection induced by the parabolic mirrors 45, 85. Thewaveplate assemblies 10, 14 include hollow shaft stepper motors withtheir rotor axes being collinear with a center axes of the reflectedbeams 42, 82, which propagate parallel along the inclining path segments46, 86. The highly compact size of the waveplate assemblies 10, 14accomplished by the use of hollow shaft stepper motors contributessignificantly to the reduced space consumption of the optical assembly12 and its successful integration into the apparatus 1 despite theincreased number of optical components compared to that of aconventional apparatus having only a single focused beam.

Referring back to FIG. 4A, 4B, a polarizer 47 is placed along each pathsegment after the first waveplate assembly 10. In the preferredembodiment, the polarizer 47 is a Rochon prism. The spatially orientedmirror 33 reflects the incoming beam and directs it along the first beampath within the horizontal beam plane.

According to FIG. 5A, a second structure 11 has an orientation on thewafer top 9 requiring a second impinging direction provided by thesecond focused beam 80. This orientation may be the result of the factthat the wafer is provided with structures having different measurementorientations in a single quadrant or the same measurement orientation ina neighboring quadrant. In the latter case, a structure with the sameorientation on a neighboring quadrant of the wafer would need to bemeasured with the second SE path, since it would require a 90 degreestage rotation to get it into the area where it can be reached by thebeam.

In order to perform the reflectance measurement, the second focused beam80 impinges the second structure 11 within the second focal spot 81along the second impinging direction and initiates a second reflectedbeam 82 that carries reflectance information away from the focal spot81. The second focused beam 80 is provided by a second lens unit 77 anda second polarizer 75 that focus and polarize the initial light beam 30propagating along the path section 71. In the preferred embodiment, thesecond lens unit 77 is similar to the first lens unit 37 and the secondpolarizer 75 is the similar to the first polarizer 35. A mirror 72 isspatially oriented to redirect the propagating beam from a horizontalbeam plane towards the angulated oriented and lower positioned polarizer75 and lens unit 77.

The second reflected beam 82 is reflected by the parabolic mirror 85from which it propagates parallel along the second inclining pathsegment 86. A polarizer 87 is placed after the second waveplate assembly14 along the path segment 86. In the preferred embodiment, polarizers87, 47 are similar. A mirror 73 is spatially oriented to direct theincoming beam along the second beam path within the horizontal beamplane.

The second moveable mirror 54 is in an in-position where it interfereswith the beam traveling along the end section 70. The second moveablemirror 54 reflects the beam again towards the parabolic mirror 56 andcontinues as described under FIG. 4A.

The perspective views of FIG. 6 and FIG. 7 illustrate the extent towhich the compactness of the waveplate assemblies 10, 14 contribute tothe small scale of the optical assembly 12 especially in the proximityof the focal spots 41, 81. For the purpose of clarity, FIG. 6 shows thefirst beam path and FIG. 7 shows the second beam path. FIG. 6illustrates the limited space available for the waveplate assembly 14between the path segment 46 and the polarizer 87. FIG. 7 illustrates thelimited space available for the waveplate assembly 10 between the pathsegment 86 and the polarizer 47.

Referring to FIG. 8, the travel envelope 90 primarily defines thefootprint of the apparatus 1 and consequently the available space on theapparatus top 8 as already explained in the above. Where the apparatus 1is configured for measuring a wafer 2 having a diameter WD of about 300mm, the envelope extensions EX, EY are according to the abovedescription about 450 mm in each direction. The travel envelope 90preferably remains within the overall boundaries of the apparatus 1 thusprimarily defining a width FY (see FIG. 3) and a depth FX (see FIG. 3)of the apparatus 1. However to allow this instrument to be used inapplications where smaller samples (e.g. 200 mm wafers) are used, whilestill requiring a minimal footprint, this instrument was designedminimizing the size of the optics plate 8 (FIG. 7) and allowing a 300 mmwafer to extend outside of the enclosure 5 (FIG. 7) in certainsituations.

Other factors like, for example, structural requirements, additionalspace for cabling and other well known components of an ellipsometricapparatus are secondarily defining the footprint of the apparatus 1.Other well known components may be part of the base 5 and/or the top 8.Such components may be required, for example, for controlling,processing, calibrating and/or focusing during the operation of theapparatus 1. Some of these components, like for example, a focusing unitand or a calibration unit may be placed on the top 8, which mayadditionally reduce the space available for the optical assembly 12.

The sectors 91–94 (FIG. 10) are predetermined and fictive areas on thewafer 2, which can be accessed for measurement without rotating thewafer 2. In the preferred embodiment and in accordance with thepreferred travel ranges TX, TY, the sectors 91–94 are about one quarterof the wafer 2. Hence, the wafer 2 has to be rotated four times in acase where all four sectors 91–94 are accessed for reflectancemeasurements.

Prior to performing a measurement, the wafer 2 is loaded on the rotatingstage 7 by a wafer-loading tool like, for example, a robotic arm thatholds the wafer 2 on its bottom surface via a vacuum fixture andreleases the wafer 2 on the rotating stage 7. The gap height HG isselected to provide sufficient space for loading and unloading of thewafer 2 even when a pin lifter assembly is used to raise the wafer toallow the robot arm to slip underneath and pick up the wafer. Due toeventual loading inaccuracies or other limitations in the loading cycle,the wafer 2 is typically globally reoriented such that the orientationof the structures 6, 11 corresponds to impinging directions. Therotating stage 7 may be configured to perform such initial globalorienting. In this regard, a flat or notch finding procedure may beperformed followed by a mask alignment procedure using a patternrecognition system.

The optical geometry relevant in that context includes an angle ofincidence IA and a focusing angle FA (see FIGS. 9, 10) of the focusedbeams 42, 82. In the preferred embodiment, the angle of incidence IA (inthe Figures, defined with respect to the surface of the wafer) is about25 degrees. The focusing angle FA or cone is between 1–6 degrees. Thereflected beams 42, 82 have a corresponding reflecting angle RA and aspreading angle SA.

As illustrated in FIGS. 6, 7, the sizes and positions of the lens units37, 77 and the parabolic mirrors 45, 85 are influenced by the angles IA,FA, RA, SA in combination with the gap height HG. Sizes and positions ofthe lens units 37, 77 and the parabolic mirrors 45, 85 again define theavailable space for dimensioning and positioning the waveplateassemblies 10, 14. The use of hollow shaft stepper motors significantlyassists in down scaling the waveplate assemblies 10, 14 so that they arepositioned along the inclining path segments 46, 86 without interferingwith the lens units 37, 77.

The waveplates are mounted in the hollow portions of the rotor shafts,and are concentrically fabricated relative to the rotor axes. Theabsence of separate waveplate bearings and a mechanical transmissionsystem greatly simplifies the design and provides at the same time for amore accurate rotation control of the waveplates. Since the rotorbearing of the stepper motor is also the bearing for the waveplates,specific bearing tolerances and tolerances for concentricity of thehollow portion of the rotor shaft are defined to meet the precisiondemands of optical assembly 12. In the preferred embodiment, linearactuator stepper motors from Eastern Air Devices were used.

FIG. 11 illustrates the optical mount 102 for the waveplate. Mount 102supports stepper motor 106 having a rotating hollow shaft 108 therein.The waveplate is mounted to the end of the shaft 108 and is carriedthereby. Reflected probe beam light (42, 82) passes through the hollowshaft and waveplate and thereafter passes through the analyzer(polarizer) 47. The output of a home sensor 110 provides feedback forthe position of the hollow shaft.

Another factor for keeping the size of the optical assembly 12 to aminimum is to utilize a common light source and a single final detector61 as described above. In order to direct the initial light beam 30 andthe reflected beams carrying the reflectance information, the moveablemirrors 28, 54 have to be switched into their in-position with highestaccuracy over a high number of switching cycles. The movement andpositioning of the moveable mirrors 28, 54 are provided by actuatorunits 29, 53. In the preferred embodiment, the actuator units 29, 53linearly move the mirrors 28, 54. The actuator units 29, 53 are computercontrolled and pneumatically operated. They provide custom designedhardened steel guides to achieve position precision over a large numberof switching cycles. The precision requirements for the mirrors 28, 54in their in-positions is 0.005° angular tolerance for 1 millionswitching cycles.

A measurement of a wafer 2 within the inventive apparatus may beperformed by the following steps. In a first step, the wafer 2 isloaded, fixated and eventual globally reoriented. In a second step, thefirst sector 91 is rotated and brought within the test area TA. Then,one of the first or second focused beams (40, 80) is activated toperform the desired measurements. Conceivably, all of the areas ofinterest within one sector could be measured by only one of the twobeams. However, depending on the orientation of the structures and thetype of measurement sought, it may be either necessary or desirable touse both beam (at different times) to measure all the features ofinterest in a given sector. For example, to gain further informationabout a periodic structure, two measurements might be made, one withbeam 40 perpendicular to the periodic structure and a second with beam80 parallel to the periodic structure.

Once the measurements of the first sector 91 are completed, the rotatingstage 7 rotates the second sector 92 within the travel area TA so thatmeasurements in this sector can be obtained. After all predeterminedsectors 91–94 have been measured, the wafer 2 is unloaded from theapparatus 1.

The output from the detector 61 is supplied to a processor for analysis.The type of analysis performed is based on the type of measurement as iswell known to those skilled in the art. For example, thin filmparameters of a multi-layer structure can be characterized frommulti-wavelength reflectometric or ellipsometric data using atheoretical model and the Fresnel equations. Information about smallperiodic structures (critical dimensions) can be derived using adiffraction model including, for example, rigorous coupled wave theory.(See U.S. Pat. Nos. 5,867,276 and 5,963,329, both incorporated herein byreference.)

The scope of the present invention includes embodiments, where theapparatus 1 is configured to make reflectance measurements on workpieces having features suitable to be measured with ellipsometrictechniques as described in the above. Further more, the inventionincludes embodiments, where the work piece is non circular, and at leastone of the ranges TX, TY is less than a parallel width of the fixed workpiece.

In the preferred embodiment, two separate beam paths are provided withina single photodetector system as exemplarily illustrated by the finaloptical elements 56, 57, 58, 59 and the final detector 61. Nevertheless,the scope of the present invention includes embodiments in which twophysically separate photodetector systems may be provided. In suchembodiment, each of the two separate photodetector systems receives oneof the two reflected beams 50, 70 thus further reducing the number ofoptical elements in the path of the reflected beams 50, 70. No moveablemirror 54 is present is such embodiments. Also, the scope of the presentinvention includes embodiments, where two separate light sources asexemplarily illustrated by the elements 20–25 are provided. In suchembodiments, no moveable mirror 28 is present.

It should be recognized that a number of variations of theabove-identified embodiments will be obvious to one of ordinary skill inthe art in view of the foregoing description. Accordingly, the inventionis not to be limited by those specific embodiments and methods of thepresent invention shown and described herein. Rather, the scope of theinvention is to be defined by the following claims and theirequivalents.

1. A method for evaluating a sample, comprising the steps of: generatinga polychromatic probe beam, splitting the polychromatic probe beam intofirst and second portions, whereby the first portion is directed toreflect off the sample in a first direction and the second portion isdirected to reflect off the sample in a second direction; determiningthe change in polarization state of the first and second portions andgenerating output signals in response thereto; and evaluating the samplebased on the output signals.
 2. A method according to claim 1, wherein:determining the change in polarization state includes using a commondetector system to selectively monitor the first and second portions. 3.A method according to claim 1, wherein: splitting the polychromaticprobe beam includes directing the second portion to reflect off thesample in a second direction that is perpendicular to the firstdirection.
 4. A method for evaluating a sample, comprising the steps of:directing a first polychromatic probe beam to reflect off the sample ina first direction; directing a second polychromatic probe beam toreflect off the sample in a second direction; directing the reflectedfirst and second probe beams along a common path; selectively monitoringthe directed first and second probe beams and generating output signalsin response thereto; and evaluating the sample based on the outputsignals.
 5. A method as recited in claim 4, further comprising:generating said first and second polychromatic probe beams includesusing a single light source.
 6. A method according to claim 4, wherein:directing a second polychromatic probe beam to reflect off the sample ina second direction includes directing the second polychromatic probebeam to reflect off the sample in a second direction that issubstantially perpendicular to the first direction.
 7. A methodaccording to claim 4, wherein: selectively monitoring the directed firstand second probe beams includes using a single photodetector toselectively monitor the first and second probe beams.
 8. A method forevaluating a sample, comprising: generating a polychromatic probe beam,selectively splitting the probe beam into first and second portions,whereby the first portion is directed to reflect off the sample in afirst direction and the second portion is directed to reflect off thesample in a second direction; directing the reflected first and secondportions along a common path; selectively monitoring the directed firstand second portions and generating output signals in response thereto;and evaluating the sample based on the output signals.
 9. A method forevaluating a sample, comprising: generating a polychromatic probe beam,selectively directing the probe beam along first and second paths, thefirst path directing the probe beam to reflect off the sample in a firstdirection and the second path directing the probe beam to reflect offthe sample in a second direction; selectively determining the change inpolarization state of the probe beam along either the first or secondpath and generating output signals in response thereto; and evaluatingthe sample based on the output signals.
 10. A method according to claim9, wherein: selectively determining the change in polarization stateincludes using a single photodetector to selectively monitor the firstand second paths.
 11. A method according to claim 9, wherein:selectively determining the change in polarization state includes usingtwo physically separate photodetectors for monitoring the first andsecond paths respectively.
 12. A method according to claim 9, wherein:selectively directing the probe beam includes using first and secondmirrors, at least one of said first and second mirrors being moveable inorder to selectively direct the probe beam.
 13. A method according toclaim 9, wherein: selectively determining the change in polarizationstate includes using a photodetector having a plurality ofphotodetecting elements generating a plurality of output signalscorresponding to a plurality of wavelengths of the polychromatic probebeam.
 14. A method according to claim 9, further comprising: selectivelydirecting the probe beam from the first and second paths along a commonpath.
 15. A method according to claim 9, further comprising; moving thesample with respect to the probe beam using a moveable stage assemblypermitting both rotation and linear motion of the sample.
 16. A methodaccording to claim 9, wherein: selectively directing the probe beamalong first and second paths includes selectively directing the probebeam to reflect off the sample in a second direction that isperpendicular to the first direction.